The goal is to analyze in greater detail the molecular interactions which regulate the SOS response in E. coli. First, specific contacts between amino acids in LexA protein and basepairs in the recA operator will be determined by isolating and sequencing mutant repressors which recognize a symmetrically altered, mutant operator sequence but not the wildtype sequence. Additionally, the role of various basepaires in the two binding sites in the Co1E1 operator on the binding of repressor will be analyzed using a rapid method for isolating and sequencing operator mutants in phage M13mp8/P. Second, we will attempt to obtain evidence for an interaction between LexA and RecA proteins that triggers repressor cleavage. A battery of mutant repressors which is liikely to be deficient in such an interaction is becoming available. An attempt will be made to isolate mutant RecA proteins which promote cleavage of these mutant repressors. The sequence changes in both the lexA and recA mutants would be determined to reveal amino acid residues in the proteins that interact and other possible compensating effects. Third, the direct role which activated Rec A protein plays in SOS mutagenesis of phage Lambda will be analysed through measurement of mutagenesis in various recA mutants and attempted isolation of new mutants based upon several current hypotheses. The molecular mechanism of mutagenesis will be analyzed by direct biophysical methods to determine if DNA synthesis plays a role. Fourth, the importance of various functions in the SOS response will be studied through analysis of mutant LexA proteins which allow constitutive expression of some SOS genes while continuing to repress the others. Finally, mutants of E. coli, which show either defective or constituteive induction of SOS functions will be isolated and analyzed to assist in identifying signal molecules and regions in the RecA protein which are involved in recognizing the inducing signal.